Dating the onset and nature of the Middle Permian Emeishan large igneous province eruptions in SW China using conodont biostratigraphy and its bearing on mantle plume uplift models

摘要

The Middle Permian Emeishan large igneous province of SW China has provided the quintessential example of the phenomenon of kilometre-scale pre-emption domal uplift associated with mantle plume impingement on the base of the lithosphere. One key line of evidence is an interpreted zone of truncation of the platform carbonates belonging to the Maokou Formation that underlies the volcanic pile. Here we test this interpretation by conodont age dating the uppermost beds of the Maokou Formation in sections from Yunnan, Sichuan, Guizhou and Guangxi provinces, which span locations from the inner part of the igneous province to several hundred kilometres beyond its margins. The results show that eruptions began in the finogondolella altudaensis Zone (～ 263 Ma) of the Middle Capitanian Stage and greatly increased in extent and volume in the J. xuanhanensis Zone (～262 Ma). Pre-eruption uplift was muted, and most locations within the terrain and at many locations beyond its margins witnessed platform collapse (not uplift) with deep-water fades (radiolarian cherts and submarine fans) developing in the J. altudaensis Zone. The clearest evidence for an emergence surface occurs around the margins of the province in the J. xuanhanensis Zone. This is after the initial onset of eruptions and marks either a eustatic sequence boundary or a brief pulse of tectonic uplift contemporaneous with volcanism. As with recent studies on the basal volcanic successions of the Emeishan LIP, kilometre-scale plume-related domal uplift prior to Emeishan eruptions is not supported by these data; rather a more complex interaction between plume and lithosphere with minor localized uplift and subsidence is inferred.